You can equip up to 4 Magatama to bolster your defenses. As you proceed through the Main Quest, Kaguya will assist in crafting new Magatama for you. You need the Recipe to craft new Magatama, and they can be found by completing quests or defeating foes.

Magatama Recipes will be added to your Inventory,
and will then allow you to Craft those Magatama.

You can confirm what materials are needed for
each Magatama by speaking with Kaguya.

When you craft a Magatama, it will require certain materials.

Crafted Magatama will have a better grade as Kaguya's affection rises. Make good friends for the best result!